The start-up handles the entire sales process from picking up the goods from sellers’ homes, to photographing, selling and shipping.
The expansion will take its number of markets to 24 after it first launched in 2014 in Sweden.
Sellpy said in a statement second-hand was one of the fastest growing market segments within the fashion industry.
As consumers become increasingly conscious about the origins and sustainability of their clothes, the fashion industry is coming under scrutiny for fuelling a throwaway culture.
H&M has invested more than 20 million euros ($24.38 million) in Sellpy and owns around 70% of the company.
Sellpy said it had started a collaboration with H&M that gives it access to an H&M warehouse in Poland, as well as service around distribution, quality control of garments and order handling.
Earlier this month, Lithuania-based Vinted raised 250 million euros ($305 million) to expand further in Europe and beyond, and said the fundraising put a pre-money valuation on the business of 3.5 billion euros.